The most common foundation type in the offshore wind industry is the monopile. The foundation of a monopile is a large steel open-ended tube with diameters ranging from 2 to 16 meters, and wall thicknesses ranging from 5 to 20 centimetres. These foundation piles are installed into the ground with an impact hammer. The depth of installation typically ranges from 25 up to 40 meters into the ground. This depth is reached by striking the top of the pile with an impact hammer. By these strikes the pile penetrates the ground.
The impact hammer creates high noise levels underwater of up to 220 dB in the immediate proximity of the pile. Restrictions by several European governments have been stated on the level of this noise. The strictest regulations are the regulations stated by the German government, which allow a maximum noise level of 160 dB SEL (Sound Exposure Level) at 750 meters distance from the source. Other countries are expected to follow these requirements, which include The Netherlands, The United Kingdom, Denmark, Sweden, Norway and Belgium. In order to comply with these regulations, noise mitigation measures have to be taken. These countries have planned to install large amounts of wind turbines in the North Sea in the coming 15 years.
Several forms of noise mitigations have been developed and deployed since these regulations where stated. The most common used noise mitigations are a bubble curtain around the pile, which absorbs the sound produced by the hammer, and a noise mitigation screen, which is in fact a large round cofferdam in which the foundation pile is placed during installation, where this cofferdam is pumped dry so no direct contact between the water and the pile exist. Several other mitigation measures have been implemented, which all prevent the sound from propagating further into the water. These mitigation measures cost an average of three hundred thousand euros for each foundation pile installation. This is approximately 15% of the total foundation costs.
One attempt to not exceed the maximum sound level while installing a foundation pile includes using a vibratory hammer that is able to install a foundation pile to a certain depth without exceeding the maximum sound level. However the vibratory hammer is not capable of installing the pile to the required penetration depth of 25 to 40 meters deep. The depth which is typically reached with a vibratory hammer in the North Sea is anywhere between 5 and 20 meters. To reach the required depth an impact hammer is used after the use of a vibratory hammer. This once more requires the use of noise mitigation measures, therefore rendering this combination not profitable.
The soil resistance, which prevents the pile form penetrating the soil, has two components. The first is the resistance of the soil along the wall of the pile, outside and for an open ended steel pile also the inside. This resistance is called the ‘shaft resistance’ and is caused by the friction between the pile wall and the soil particles. The second is the resistance of the soil underneath the pile head end. When the pile penetrates the soil, the soil has to be pushed away to make room for the pile to enter. This resistance is called the ‘tip resistance’. During vibratory driving in sand, the soil type commonly found in the North Sea, the shaft resistance is low compared to the tip resistance. Friction fatigue of the soil is considered responsible for this. A vibratory hammer typically vibrates with a frequency of 10 Hz to 30 Hz and with the amplitude of the pile and hammer, which are rigidly connected, of 0 to 10 millimetres in the vertical plane. During vibratory driving the soil around the shaft is shaken by these motions and the soil experiences a high number of loading cycles, up to 1×105 to 10×105 loading cycles are applied to the pile and soil around the pile during the time it takes to install the pile. These loading cycles cause fatigue in the soil. The frictional strength reduces by 80% to only 20% of its initial value. However the soil underneath the pile at the time of installation does not experience this large number of loading cycles because the pile enters new soil every time it penetrates further into the ground. Also, the shear strength of sand is, compared to other soil types such as clay, very high which in turn causes a high tip resistance in this type of soil. The combined shaft- and tip resistance of the pile during installation is the total resistance. The majority of this resistance, in hard sandy soils, such as the North Sea, is the tip resistance. This follows from pile driving predictions and measurements taken during pile installations with a vibratory hammer. What is needed is the reduction of the high tip resistance, where the pile could be installed to the full-required penetration depth with a vibratory hammer, while meeting noise reduction requirements.